Remark 3.2 In fact, we will take (REF ) and
(REF ) as the definition of the solution to
(REF ) and (REF ), respectively. Indeed, while the
function (REF ) is given explicitly, there exists a
unique function satisfying (REF ) (see e.g. [1]}). If the solution to (REF ) and
(REF ) exist, it can be shown (see e.g. [2]} for (REF ) and
[1]} for (REF ))
that they satisfy (REF ) and
(REF ), respectively.

